1. Field of the Invention
The present invention generally relates to snowmobiles. More particularly, the invention relates to rear suspension and exhaust systems for snowmobiles.
2. Discussion of the Related Art
Snowmobiles are generally used to travel over terrain covered with snow. As shown in FIG. 1, a conventional snowmobile has several major components: a front suspension 1 with skis that are connected to a steering mechanism 2; a main body 3, including a frame which houses an engine that drives a continuous belt or track 4; a tunnel 5 that covers the upper portion of the track; and a seat 6 for an operator. A rear suspension assembly 7 is located within the area encompassed by the track.
Flexible track vehicles, such as snowmobiles, present unique suspension problems. A suspension should provide driving comfort and operating safety by absorbing uneven terrain features--this requires the suspension to have a large range of motion, or travel. In an automobile the suspension assembly allows the tire to pivot upwards and downwards, permitting a large amount of travel. In contrast, the suspension assembly for a tracked vehicle anchors the track and only allows the track perimeter to compress and expand, without any pivoting motion of the track as a whole, thus limiting suspension travel, as shown in FIG. 2.
For example, prior art suspension assemblies, as shown in FIGS. 2 and 3, are designed to guide the track perimeter into a generally parallelogram shape. An upper track routing 8, housed within the tunnel 5, is defined by a drive sprocket 11, and a set of idler wheels 12. A lower track routing 9 is defined by another set of idler wheels 12, and a pair of slide rails 13. Springs 14 prevent the track from collapsing and shock absorbers 15 dampen the springs' oscillations. Various arms 16 connect the slide rails to the tunnel.
When a snowmobile encounters a bump, the lower track routing moves upwards and the track compresses, but the upper track routing remains fixed relative to the frame 3 in the tunnel 5, as shown in FIG. 2. Once over the bump, the track perimeter expands as the lower track routing resumes its previous position relative to the frame and upper track routing. However, the compression and expansion of the track perimeter is limited because the upper and lower track routings have a substantially fixed length. Therefore, the only sections of the track that can vary in length, permitting suspension travel, are the "side" track routings 10 that connect the upper and lower track routings. This limits the total amount of suspension travel to about 10 inches. Because of this limited suspension travel, a significant amount of the force exerted by bumps is absorbed by the frame 3 and the operator, which can adversely affect operator comfort and vehicle handling.
Prior art suspensions have employed a variety of devices and concepts in a generally unsuccessful effort to keep the limited travel suspension from compressing completely. This is known as "bottoming." Stiffer springs that require more compression force can be used, but this degrades operator comfort when traversing relatively smooth terrain as the springs are too stiff to move in response to smaller bumps. Stiffer springs are also heavy and more expensive.
Another way to keep the suspension from bottoming is to increase the suspension's stiffness as it is compressed, a "rising-rate" suspension. Put differently, as the suspension is compressed, an increasing amount of force is required to compress it further. Generally, two techniques are currently used to achieve a rising-rate suspension: 1) specially-designed springs; and 2) sophisticated linkage arrangements that pivotally connect the suspension arms to the springs. Under either technique, the components are heavy and expensive. Furthermore, with the sophisticated linkage arrangements of technique (2), each pivot point experiences high forces that can cause binding, making the overall system even more stiff, and less able to follow the terrain.
Complex linkage arrangements are also used to overcome shortcomings associated with existing "suspension geometry." The "geometry" of a suspension is defined by the arrangement of the lengths and pivot points of the arms, links and other suspension components. For example, the slide rails' movement is controlled by the suspension arms pivoting about their attachment points on the snowmobile frame. One important geometry shortcoming associated with conventional suspensions is the tendency for the front of the slide rail to move independently of the rear. In other words, when initially encountering a bump, the front part of the slide rail moves upwards but the rear part does not, resulting in a tilted slide rail. When the rear part of the slide rail, now tilted, hits the bump, the force absorbed by the suspension, and the operator, is greater. Very complex, heavy and expensive suspension assemblies have been designed in a generally unsuccessful attempt to eliminate slide rail tilting. Slide rail tilting can also cause undesirable slack or tension to develop in the track. If slide rail tilting can be eliminated, springs and shock absorbers can be softer, reducing weight and cost and increasing operator comfort and safety.
Another shortcoming of conventional suspension assemblies is the difficulty associated with weight transfer. Weight can be transferred from the front of the track (that is, the portion of the track closest to the skis) to the rear of the track (that is, the portion of the track farthest from the skis) by adjusting the slide rails. A slight upward incline shifts the weight to the rear of the track, and a slight downward incline shifts the weight to the front of the track. This changes the amount of weight carried by the front suspension and steering skis. When operating on ice, it is preferable to transfer more weight onto the skis, which helps them to "bite" the ice for better control. When driving on lightly packed snow, less weight on the skis and front suspension helps the skis to ride on top of the snow. Snow conditions may change frequently, but adjusting weight transfer on conventional suspension assemblies is somewhat difficult. Therefore, operators may not always bother to optimize the weight transfer.
It is also difficult to adjust the springs and shocks on conventional suspension assemblies. Because they are located within the track, and the track becomes filled with snow during operation, adjusting the springs or shock absorbers requires laying down in the snow and removing all the snow that has accumulated around them.